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Detlef Weigel's group previously showed that endogenous plant miRNAs are much more specific than miRNAs or synthetic siRNAs in animals. To confirm that this is due to genuine differences

between the RNAi machineries in plants and animals, rather than selection against miRNAs that have broader specificities in plants, they designed a range of miRNAs to target various

endogenous genes in Arabidopsis thaliana. In cases for which the effects of mutating the endogenous target genes were already known, the phenotypes that resulted from the overexpression of

the corresponding designer miRNAs were remarkably similar to the outcomes of mutation. Weigel and colleagues also used microarrays to show that, like endogenous miRNAs, designer miRNAs

(known alternatively as artificial or synthetic miRNAs) are highly specific — they knock down their predicted targets, but little else. This confirms the intrinsic specificity of

miRNA-mediated silencing in plants and highlights designer miRNAs as potentially useful tools.

In independent work, Yuval Eshed and colleagues found that designer miRNAs work not only in A. thaliana, but also in other species including tomato and tobacco — a finding that is perhaps

unsurprising given the considerable conservation of some miRNAs. This highlights the potential for this new tool to be applied to plant species that are not normally amenable to genetic

manipulation.

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